For a wide-angle lens for digital cameras, there is increasing demand toward an optical system having a long back focus, because of the need for providing a space for receiving optical elements such as filters just in front of an electronic imaging plane while making sure a wide angle of view. As such an optical system, there has been a retrofocus type optical system widely adopted, which comprises a front unit of negative refracting power and a rear unit of positive refracting power. Upon incidence on the electronic imaging plane of a light beam passing through the optical system, the quantity of rim light decreases, giving rise to shadings or color shifts, at a time when the incident light beams enter obliquely with respect to the optical axis (the so-called oblique incidence). To solve these problems, there has been an optical system proposed, which keeps hold of the telecentric feature of the incident light beam. Such optical systems have been proposed in JP(A)'s 2010-61033, 2003-241084 and 2000-235145.
A wide-angle lens for recently developed digital cameras is now required to have a low lenses count. With fewer lenses, however, it is difficult to design an optical system that is reduced in terms of aberrations while making sure any desired wide angle, and reduced in terms of field curvature fluctuations upon focusing while keeping the telecentric feature intact.